The present invention relates to electrical generator arrangements and particularly to generator arrangements which draw power from a generator having a large range of operating speeds, during use. Applications of the invention are envisaged in the aerospace field, for instance.
Gas turbine engines are conventionally used as aircraft engines to provide propulsive thrust for the aircraft. In addition, gas turbine aircraft engines are required to provide a source of electrical power for accessories on the engine and on the aircraft upon which the engine is operationally mounted. The conventional way of extracting electrical power from a gas turbine engine is to provide a mechanical drive from a compressor shaft of the engine, through a suitable gearbox, to the generator. The gearbox and generator are usually mounted on or within the fan casing of the engine or on the engine core. The presence of these components adds undesirable weight and complexity to the engine.
Permanent magnet generators have been considered for use in these situations. They are a well understood design of generator for which various magnetic configurations have been proposed. They can be robust and reliable in use. However, the EMF (electro motive force) of a permanent magnet generator is proportional to the speed at which the generator rotor is rotated. The range of rotation speeds within a gas turbine aircraft engine can be considerable, varying between the rotation speed when idle on the ground, and the maximum speed during take-off. However, the generator arrangement is required to deliver constant and relatively low voltages to the systems being supplied. The generator may also be required to be xe2x80x9cfault tolerantxe2x80x9d. That is, the generator may be required to have sufficient inductance to ensure that when the terminals are short-circuited at any generator speed, the short circuit current does not exceed the maximum rated current.
The invention provides an aerospace generator arrangement operable to provide DC power from a rotating member, the arrangement having a permanent magnet generator driven, in use, by the rotating member to provide AC power at the output terminals of the generator, converter means operable to convert the AC output to provide the DC power, and control means which, in use, maintain constant AC voltage at the generator output terminals, regardless of the generator EMF.
The generator is preferably fault tolerant. Preferably the control means comprise a voltage synthesiser arrangement operable to impose a synthesised voltage waveform on the generator output terminals. The voltage synthesiser may be a pulse width modulation-type synthesiser device. The voltage synthesiser preferably creates the synthesised voltage waveform from the DC power.
The rotating member is preferably driven from an engine and may be driven from a compressor, shaft of a gas turbine engine, preferably from the low pressure shaft of a multi-shaft gas turbine engine.
The invention also provides an aerospace generator arrangement operable to provide DC power for use within an aircraft, the arrangement comprising an aero engine which drives a permanent magnet generator to provide AC power at the output terminals of the generator, converter means operable to convert the AC output to provide the DC power, and control means which, in use, maintain constant AC voltage at the generator output terminals, regardless of the generator EMF.
Preferably, the aero engine has at least one rotary shaft connecting a turbine and a compressor, the generator being driven by or from the said shaft. Preferably the engine has more than one shaft. The driving shaft may be the low pressure shaft.
The control means may comprise a voltage synthesiser arrangement operable to impose a synthesised voltage wave form on the generator output terminals. The voltage synthesiser may be a pulse width modulation-type synthesiser device. The voltage synthesiser may create the synthesised voltage waveform from the DC power.
In a further aspect, the invention provides an aircraft incorporating a generator arrangement as aforesaid.